1. Field of the Invention
The present invention relates to a color reduction processing apparatus and method for converting color image data containing many colors to print image data containing fewer colors. The invention also relates to a printer control apparatus and method for printer control according to the type of printer by converting and outputting control commands and print data from a host system according to the type of printer, and more particularly for converting image data containing multiple colors, such as full-color image data, to print image data containing a first color, a second color, and a non-printing color.
2. Description of the Related Art
Because printers used in such commercial applications as POS (point-of-sale) systems and ATMs (automatic teller machines) print primarily letters, numbers and symbols, such printers are typically black and white monochrome printers that print black letters. Some recent POS systems, however, use color printers that can print both a primary print color, usually black, and an auxiliary print color such as red. This primary print color is referred to below as the “first color,” the auxiliary print color is referred to as the “second color,” and this type of color printer is referred to as a “two-color printer.” Printing a logo, advertisement, or coupon, for example, using two colors of ink can improve the visual effect. A two-color printer is capable of three-color expressions, that is, the first color, the second color, and the non-printing color, which is the color of the print medium (paper). Gray scale printing in which the color density of the first and second print colors is varied in different levels is also possible by changing the dot density per unit area. This ability to print two colors in combination and to vary the density of each printed color enables a two-color printer to print relatively sharp color images using two print colors without particularly detracting from the original image, which may be a photograph or picture.
Image data from photographs and pictures typically contains many colors. These colors are often defined in the RGB color space using combinations of red (R), green (G), and blue (B). Image data for a full-color image in the RGB color space can be expressed using 256 levels (0 to 255) of color depth for each color, that is, red, green, and blue. Full-color image data can be reduced to image data for printing using fewer colors by various methods, including error diffusion, dithering, and halftoning. Direct application of these methods alone, however, often does not produce a satisfactory replica of the original full-color image using fewer colors. To solve this problem, methods of reducing full-color image data to eight colors by dithering, and then using the luminance value of each resulting pixel to convert the 8-color image data to three-color print image data, are provided in JP-A-2002-269550, JP-A-2002-288682, and JP-A-2002-314833.
While these techniques can use various processes to manipulate the image data, such techniques also present problems. More specifically, the reduced-color image output by the color reduction process greatly deviates from the original color image because the colors in an image expressed in the RGB color space, which is a three-dimensional space that is difficult to grasp visually, are reduced based on the individual RGB elements. The foregoing prior art also requires a two-part process, that is, reducing the full-color image data to eight colors and then reducing the eight colors to three colors (two print colors and the non-printing color of the paper), thus making the color reduction process and operation more complicated.
Furthermore, because the final output color in the RGB color model is determined by the individual color depth of the overlapping R, G, and B colors, which color should be specified at which threshold value is difficult to determine visually. As a result, the operator must incrementally adjust the color reduction threshold values while confirming the effect of the threshold values on the image after color reduction processing in a preview or test screen. The color reduction process is thus often time-consuming. What colors can be printed by the printer must also be defined as parameters for any color reduction process, but setting these color parameters is both a nuisance for the operator (user) and prone to error.